1. Field of the Invention
This invention relates generally to casting and, more particularly, to improvements in closed risers which produce a cleaner more solid casting, as well as a small contact area between the riser and the cast article.
2. Description of the Prior Art
The concept of using a stricture or collar formed between the riser and the cast article is shown in the Nieman et al U.S. Pat. No. 3,831,662. Briefly, a frangible nonfusible constricting means is placed at the juncture of the riser port and the mold cavity. The constricting means is provided with a relatively thin apertured wall which conforms to at least one dimension of the mold cavity. The wall is of sufficient thinness with respect to the high thermal conductivity of the wall material to prevent loss of heat with resultant premature cooling and hardening of the molten metal at the juncture. In the Nieman embodiment the collar is placed between the riser and the cast article where the riser extends downward below the cast article.
A second type of riser liner is shown in the Washburn U.S. Pat. No. 900,970 in which a clay pot having a construction is used as a liner for an open riser. In the Washburn patent the open riser is located above the cast article with the frangible material extending along the sides and bottom of the riser.
A prior art closed feeder head is shown in the Krzyzanowski U.S. Pat. No. 3,295,175. Krzyzanowski shows a spherical feeder head made of a cellular material which mounts on a peg in the wood pattern. The spherical feeder head is made of expanded cellular polystyrene. When the hot metal contacts the polystyrene, the polystyrene gasifies producing space for expansion of metal into the riser during the casting process. One of the problems with the cellular spherical feeder is that sand particles become embedded in the cellular material during molding. During gasification of the cellular material, the same particles are freed and begin to work their way down into the casting thus weakening the casting.